A. Field of the Invention
The present invention relates to a new and improved assembly for increasing engine stability and to a new and improved method therefor; and more particularly, to a new and improved load sensitive fuel cutoff for imposing a load on a diesel engine in an engine-generator by cutting off fuel to one or more engine cylinders and to a method for cutting off fuel to one or more cylinders in a diesel engine at predetermined loads on the engine thereby increasing engine stability at no load and light load situations.
B. Description of the Background Art
In certain situations such as hospitals, nursing homes, construction sites, and recreational vehicles an auxilliary source of electricity is necessary. For example, in hospitals standby electrical power provided by engine-generator sets which include a diesel engine and a generator are necessary to provide electricity in emergency situations when normal power has been interrupted. In situations in which a source of electricity is not readily available such as at construction sites and with recreation vehicles, an engine-generator set provides necessary electricity to run equipment.
In these and other uses of engine-generator sets engine stability is an absolute necessity to maintain the frequency of the alternating current produced by the generator constant. An engine is stable while it runs at a constant rpm. In an unstable mode the engine wanders from a constant rpm in a cyclic hunt for operating speed. Engine instability is particularly undesirable in situations requiring a plurality of parallel engine-generator sets. For example, hospitals require large power backup of a magnitude not available from a single engine-generator set. To have available the necessary emergency power, more than one engine-generator set is connected in parallel. When auxilliary or emergency power is required, each engine-generator set is started and they must quickly reach the same frequency, phase, and speed in order to be coupled to a buss. Existing regulations require all engine-generator sets to come on line within ten seconds. If one or more of the engine-generator sets includes an unstable engine, that engine will unsuccessfully hunt for the operating speed preventing the engine-generator sets from coming on line.
A desired procedure for improving engine stability is to improve the combustion process of the engine. Recently, however, it has been desireable to obtain increased horsepower from engines for the same displacement by increasing fuel consumption and decreasing compression ratios. With reduced compression ratios, heat generated in the engine at start up is low. Low heat results in low engine temperature and poor combustion which causes engine instability. One way to avoid engine instability at start up is to preheat the engine but it has proven difficult to heat engines sufficiently to overcome this problem.
It is also typical with engines in generator sets to have instability at operating speed with no or little load. These engines are designed to operate under a load and if there is no or little load at operating speed, the engine does not heat up to the preferred level. The low heat results in low engine temperature and poor combustion causing engine instability.
It has been determined that engine instability at start up and at operating speed with no or little load can be corrected if a load is applied to the engine. By applying a load the engine heats up quickly and runs hotter resulting in improved combustion and engine stability. The most direct procedure for imposing a load on an engine is to stop the flow of fuel to one or more of the cylinders. The inactive cylinders act as air compressors and consume work whereas a fueled or active cylinder creates more work than it consumes. The additional work imposed on the fueled cylinders increases engine temperature improving combustion and allowing stable engine operation.
Several approaches to impose a load on an engine have been unsuccessfully attempted. One approach employs a check valve in one or more fuel lines of the engine. At low engine speeds the check valve prevents fuel flow to one or more of the cylinders. A check valve has been found to be too restrictive, and in generator sets it has been found that the check valve does not allow sufficient fuel flow to the active cylinders.
Another approach involves a pressure sensitive switch in one or more of the engine fuel lines. These switches measure pressure of the fuel from the fuel pump. Switches of this type have proven unsatisfactory because when the fuel lines to the inactive cylinders are opened, the change in fuel pressure causes the switch to cycle resulting in engine instability.
There is a substantial need to introduce a load onto an engine in an engine-generator set that will increase engine stability during engine start up and at operating speeds with no or little load.